Heretofore, electric vehicles have been known that travel based on a rotational torque transmitted to road wheels through a power transmitting mechanism from an electric motor that is energized by electric power from an electric storage device. Problems to be solved in order for such electric vehicles to find widespread use include the relatively small distance that such vehicles can travel with a single charging process performed on the electric storage device, lack of pervasive charging facilities, and long charging times, etc.
Another concern in relation to electric vehicles is that, since electric accessories such as an air conditioner, etc., consume electric power, if the electric vehicle is involved in traffic congestion, then the distance that the electric vehicle can travel tends to be extremely reduced.
Recently, a wired charging process has been proposed for charging the electric storage device on an electric vehicle through a charging cable that interconnects a charging plug on the electric vehicle and a charging stand or a home power supply. If the electric vehicle is to be charged on a daily basis, then it is highly tedious and time-consuming to connect and disconnect connectors on the charging cable.
There have been demands for wireless charging, or stated otherwise, contactless charging, in order to eliminate the above tedious and time-consuming process.
Japanese Laid-Open Patent Publication No. 2009-106136 discloses an electric vehicle (electrically-driven vehicle) that receives charging electric power contactlessly from a power supply outside of the electric vehicle according to a resonance process, which is used to charge an electric storage device in the electric vehicle. Japanese Laid-Open Patent Publication No. 2009-106136 states, in paragraphs [0015], [0033], etc., that a secondary self-resonant coil, which is an LC resonant coil having both ends thereof open, should preferably be disposed in a lower portion of the vehicle body. The patent publication also states (FIG. 1, paragraph [0034]) that a secondary coil (electric receiving coil) should preferably be disposed coaxially with respect to the secondary self-resonant coil (paragraph [0092], FIG. 10), which is disposed in the lower portion of the vehicle body parallel to the electric vehicle.
Wireless coupling (wireless electric transmission) according to a resonant process and wireless coupling according to an electromagnetic induction process are disclosed in “Second Section, Medium-Distance Electric Trans-mission Technology,” pages 27 through 31, of “EE TIMES Japan” published Oct. 9, 2009 by E2 Publishing Company (hereinafter referred to as “Non-Patent Document 1”), for example.
Non-Patent Document 1 reveals that if ω represents an angular frequency, Rohm represents a resistive component, and Rrad represents a radiation resistive component, then a coil index Q concerning energy retained by the coil is expressed by the following equation (1).Q={ωL/(Rohm+Rrad)}  (1)
Non-Patent Document 1 also discloses that if the inductance of an electric transmitting coil is represented by Ls, the inductance of an electric receiving coil is represented by Lr, and the mutual inductance is represented by M, then the coupling strength k between the coils is expressed by the following equation (2).k=M/(Ls·Lr)1/2=(M/√Ls·Lr)  (2)
Non-Patent Document 1 further discloses that a figure of merit concerning the electric power transmission efficiency of a wireless coupling is expressed as the product of the coupling strength k and the index Q, in accordance with the following equation (3).k·Q=(M/√Ls·Lr)·{ωLs/(Rohm+Rrad)}  (3)